The present invention relates to pellicles that are used in the semi-conductor chip industry, and more particularly to an adhesive system for reducing the likelihood of particulate.
In the semi-conductor chip industry it is well known that pattern transfer from the photomask to substrate is accomplished by exposing a mask to a light source. During the pattern transfer process, also called the photolithographic process, patterns on the photomask are projected onto the substrate which has been treated with a photosensitive substance. This results in the mask etchings being reproduced onto the substrate. Unfortunately, any foreign substance on the surface of the mask will also be reproduced on the substrate and therefore will interfere with proper pattern transfer to the substrate.
To eliminate contamination of the mask surface, a framed, thin membrane known as a pellicle is mounted on the mask surface, such that the pellicle membrane extends parallel to the mask at a predetermined distance spaced away from it. Any contamination which would ordinarily land on the mask surface instead falls onto the pellicle membrane.
Pellicles substantially eliminate the above problem because contamination on the pellicle membrane will not be projected onto the substrate. The frame of the pellicle supports the membrane at a distance spaced away from the mask surface so any particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer.
The use of pellicles can increase the quality of the resulting circuit, thereby dramatically improving circuit fabrication productivity. Consequently, it is no surprise that pellicle manufacturing techniques have become increasingly important because high quality pellicles are critical to the success of the photolithographic process.
During the pellicle manufacturing process, it is important to minimize the possibility of either relatively large or small contaminant particles being deposited on the pellicle membrane. Relatively large particles are unacceptable because they may be reproduced in the substrate during photolithography even though they are out of focus. Equally unacceptable are particles (whether large or small) that are deposited on the underside of the pellicle membrane or the pellicle frame. Such particles may drop onto the mask surface during photolithography which is precisely what is to be avoided by using pellicles.
It is also critical that the pellicle membrane be extremely uniform across its surface. This is so that the light passing through the membrane during lithography is unobstructed and is not in any way refracted. Thus, the composition of the membrane must be highly uniform, and the membrane must be evenly tensioned across the pellicle frame. Also, it is important to ensure that a continuous seal exists between the thin film and the frame.
To further understand these important requirements, it is necessary to provide an explanation as to how pellicles are formed.
As is known in the art, forming an optical membrane is the first step in pellicle manufacture. Commonly the membrane is prepared by spinning a suitable polymer, such as nitrocellulose or a nitrocellulose-containing polymer, on a substrate. The newly formed membrane is then removed from the substrate and held under tension, adjacent its periphery, to prepare it for subsequent manufacturing steps.
Next, a frame is fastened, or bonded, to a working area of the membrane, framing the working area. After fastening, the frame and the working area of the membrane attached to it are ready to be separated from the remaining area of the membrane.
As suggested above, it is particularly critical that no particulate be generated by the underside of the membrane and the inner sides of the pellicle frame. This is because any particulate that falls from these surfaces will fall directly onto the photomask, and during photolithography is likely to be reproduced on the wafer substrate. One problem which exists with some of the prior art designs is that various adhesive surfaces terminate at edges or at surfaces which may result in the generation of particulate at such edges or abutting surfaces.
It is therefore an object of the present invention to develop a pellicle system which will be far less likely to generate particulates in the region defined between the underside of the pellicle membrane and the inner side of the pellicle frame. An additional object of the present invention is to develop such a system which is relatively simple and therefore inexpensive to fabricate, and which does not have a deleterious effect upon the performance of the pellicle. As noted above, it is critical that pellicle membrane transmissivity and uniformity be substantial, so it is important that the objects of this invention be achieved without adversely affecting the performance of the pellicle.